1. Field of the Invention
This invention relates generally to fluid bed reactors and, in particular, to fluid bed reactors having a grid baffle structure in the reactor which divides the reactor into reaction stage sections through which feed to the reactor flows sequentially through the reaction stage sections from the inlet of the reactor to the outlet of the reactor and which baffle structure increases the staging of the reactor and therefore provides a relatively uniform average particle retention time in the reactor. The reactor is especially suitable for performing time/temperature dependent chemical reactions such as the reduction/reaction of iron ore to iron carbide.
2. Description of Related Art
Fluid bed reactors are well known and have been applied to a number of industrial processes. The reactors may be used as chemical reactors for endothermic reactions such as the calcination of phosphate rock and the reduction of iron ore. Exothermic reactions such as roasting of zinc sulfide ores may also be performed in a fluid bed reactor. The reactors are also used as dryers for drying granular materials such as sand, plastics, detergents and the like. Combustion fluid bed systems have been used for the closely controlled disposal of toxic liquid and semi-solid wastes such as activated sludge, pulping waste and other organic wastes. The reactors are also used as boilers and steam generators and such reactors include fluid bed boilers, package and field erected boilers, superheaters and air heaters. The reactors can be used on a wide variety of feeds including coal, oil, gas and industrial and home refuse.
Fluid bed reactors typically comprise a vertical vessel having a lower horizontal plate, i.e., an air distributor or constriction plate, which supports a bed of particulate solids in a reaction chamber and separates the reaction chamber from a windbox or plenum below the plate. Combustion air is introduced into the windbox and passes upward through openings in the air distributor plate in sufficient volume to achieve a gas velocity that expands or fluidizes the solids, suspending the particulate solids of the bed in the flowing air stream forming a fluid bed and imparting to the individual solid particles a continuous random motion. Some important advantages of conducting a reaction or other heating or cooling process in a fluidized bed includes the substantially uniform bed temperature, combustion at relatively low temperatures and a high heat transfer rate.
The suspension of solids by an upward gas stream in the fluid bed reactor resembles a bubbling fluid, i.e., boiling water. The suspension is typically contained in the lower-middle portion of a vertical reactor termed a reaction chamber and is bound laterally by the reactor walls and below by a constriction plate beneath which is the windbox. The top of the bed looks like an irregular splashing, boiling surface and turbulence in the bed and intimate solid to gas contact provides a near ideal environment for carrying out chemical reactions, heating, cooling, combustion and other unit operation processes. The fluid bed for many operations is typically silica sand or other inert material and provides the area in the reactor in which most of the fluid bed process reactions occur. The bed may also be materials which are converted such an iron ore to iron carbide.
The intimate mixing in the bed provides high reaction kinetics and/or heating and/or combustion which increases process efficiency and enhanced energy utilization. The high turbulence and relatively long gas residence times of 5-8 seconds or more is satisfactory for most operations such as calcination, incineration and the like since for these reactions, the reaction kinetics are very rapid. The absence of interior moving parts in the reactor also provides minimal maintenance requirements. The bed provides a thermal fly wheel which minimizes temperature fluctuations due to variations in the feed and permits intermittent or batch operation with little or no auxiliary feed and avoids thermal shock to the refractory system. Capital and operating and maintenance costs are substantially less than for comparable rotating furnaces or multiple hearth furnaces. Staffing is usually one operator per shift or less.
As with all systems utilizing turbulent flow there is a natural back mixing in fluid bed reactors which enhances thermal efficiency. For many reactions the kinetics are very rapid and there are no negative aspects of back mixing. For time/temperature dependent reactions however, high conversion or completion of reaction are not feasible in the conventional fluid bed reactor which is considered a single stage reactor. Backmixing provides a non-uniform bed solids residence time profile. Particles which have short residence times are incompletely reacted. Excessive residence time has no beneficial effects. To increase the staging, multiple fluid bed reactors with series solids flow (gas flow either countercurrent in series or parallel) or parallel baffles in the reactor (sometimes radial baffles), have been used to generate a more uniform solids residence time profile closer to plug flow rather than turbulent flow. When residence time in the reactor is a factor in the process being performed and/or a great number of stages of operation are required, classical methods of staging using multiple reactors and/or increasing the staging in a single conventional fluid bed reactor, are technically not viable and/or are economically unattractive.
Time/temperature dependent reactions as noted above, are not feasible in a single stage fluid bed reactor and a number of processes and specially designed fluid bed reactors have been used in the past. A particularly important time/temperature dependent reaction is the reduction and reaction of iron ore (such as iron oxide) to produce iron carbide which may then be more easily used to produce iron or steel and the following description will be directed to this reaction although it will be appreciated that the invention is useful for other such time/temperature dependent reactions needing multiple staging.
U.S. Pat. No. 4,082,545 shows the direct reduction of iron ore in a system of multiple fluidized bed reactors to which the iron ore is successfully fed in series with the result that in each reactor, the iron ore is progressively reduced until a desired reduction of about 95% is achieved in the final reactor. The reducing gas is introduced into the bottom of each of the fluidized bed reactors and flows countercurrent to the direction of iron ore flowing through the reactors.
In U.S. Pat. No. 3,985,547 iron ore is reduced in a single fluid bed reactor wherein multiple vertical superposed bed levels are provided in the reactor tower and the ore passed by gravity downwardly from bed level to bed level. The lowermost and all superposed beds are fluidized by off gasses from which carbon dioxide and water have been stripped and by combusting methane with a substoichiometric quantity of oxygen and introducing the combustion products into an intermediate level of the column. U.S. Pat. No. 3,236,607 relates to a process for reducing iron ore to metallic iron in multiple stages in a single fluid bed reactor wherein the reactor is separated into two or more stages or chambers by placing one or more vertical transverse baffles extending across the diameter of the reactor and providing means for the transfer of solids from one side of the baffles to the other side by gravity with minimal back mixing of the solids.
U.S. Pat. No. 3,672,069 shows a fluidized bed cooler in which a plurality of parallel compartments are arranged side by side and are separated by respective baffles forming a weir or overflow arrangement whereby particles move from one compartment to another by gravity overflow and each compartment is fluidized by the introduction of a carrier or fluidizing gasses from below.
U.S. Pat. No. 5,406,718 shows an apparatus for drying particulate materials such as seeds and grains in which an air drier having superposed vertically spaced drying stages having mesh openings at least equal or larger than the suspension speed of the particles to be dried so that the seeds are temporarily suspended by the drying air flow until enough are accumulated to provide sufficient weight or downward pressure for the particles to overcome the upward air flow and pass downward through one or more mesh screens and reach the bottom of the dryer.
U.S. Pat. No. 5,184,671 shows a fluidized bed heat exchanger having a plurality of chambers formed in the housing wherein the chambers communicate to permit the material to flow from the feed inlet to at least one chamber and eventually out of one of the chambers and from the exchanger.
U.S. Pat. No. 4,021,184 is of interest to show a fluidized bed waste incinerator which uses cyclones to return particulate solids to the reactor which solids have been carried out of the reactor with the exhaust gases.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a fluid bed reactor having a number of theoretical stages in a single reactor for efficiently and effectively performing time/temperature dependent reactions and other processes requiring an increased number of reaction or conversion stages in the reactor.
It is a further object of the invention to provide a method for performing time/temperature dependent reactions in a single fluid bed reactor.
Other objects and advantages of the present invention will be readily apparent from the following description.